The present invention relates to an air-fuel ratio control system for an internal combustion engine, which system controls the air-fuel mixture to the stoichiometric air-fuel ratio, at which ratio a three-way catalyst acts most effectively.
In a known air-fuel ratio control system for a motor vehicle, the air-fuel ratio of the air-fuel mixture burned in the engine cylinders is detected as the oxygen concentration in the exhaust gases by means of an O.sub.2 sensor provided in the exhaust system of the engine, and a decision is made dependent on the output signal from the O.sub.2 sensor which indicates whether the air-fuel ratio is richer or leaner than the value corresponding to the stoichiometric air-fuel ratio for producing a control signal. The control signal is applied to a proportion and integration circuit (PI circuit), the output of which is changed to pulse form. The pulses operate an electromagnetic valve so as to control the amount of bleed air in the carburetor. The duty ratio of the pulses is changed in accordance with the output of the PI circuit for controlling the air-fuel ratio of the mixture. When the duty ratio of the pulses is reduced, the air-fuel mixture is enriched. Thus, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio, at which ratio a three-way catalyst in the exhaust system acts most effectively.
On the other hand, generally the carburetor does not have a flat load chateristic. Namely, the carburetor supplies richer air-fuel mixture at light load, and supplies lean mixture at heavy load.
In addition, when the engine at heavy load is rapidly decelerated, high intake manifold vacuum occurs, and fuel adhering on the wall of the intake manifold is sucked into the cylinders together with the intake air at a high vacuum. Accordingly, the air-fuel mixture is temporarily enriched, increasing the oxygen concentration in the exhaust gases. The emission control system operates to increase the duty ratio (for example 80%) so as to dilute the rich mixture, which results in an extremely lean mixture supply. FIGS. 4(a) and (b) show an increase of the duty ratio and the lean air-fuel mixture by dotted lines. Under such a condition, if the engine is re-accelerated to a heavy load range (lean mixture supply range), the air-fuel mixture is more diluted. Accordingly, the driveability of the vehicle is very low in such driving conditions.